As a result of changes in design code requirements for wind loads, increased demand for electrical power, and the growing communication industry, retrofitting of existing steel lattice towers has become an important strategy in the transmission tower industry. At present there are two main reinforcing methods that are used in practice for lattice transmission towers. The first method involves adding a number of horizontal braces (diaphragms) to the mid-length of slender, inadequately braced members along the height of the tower; while the second method is focused on strengthening the capacity of the existing legs only by retrofitting them with additional members. In this paper, the most effective leg retrofitting method has been investigated through an experimental testing program using a one-panel angle leg retrofitting model and a non-linear finite element (FEM) model. A 3D continuum element was developed to model the panels and bolted connections and the results are compared with those obtained from experiments, with good agreement. The model was then simplified using a simple locking system where a special technique was developed to simulate the bolt slip, which was found to be a critical issue from the experiments. The simplified model can then be extended to multi-panel modelling.